The present invention relates to nuclear reactor fuel assemblies, and in particular to apparatus for holding a fuel assembly down against a core support stand.
It is known that the coolant flow rate in modern nuclear reactors can be so large that the weight of the fuel assembly is insufficient to prevent the assembly from being lifted off the core support stand. One solution is to place coil springs at the top of the assembly and use the fuel upper alignment plate to compress the springs and bias the assembly down against the support stand. Although this arrangement is effective in holding down the assembly, the downward biasing force places a compressive load on the fuel assembly skeleton, i.e. the control rod guide tubes, and is believed to contribute to bowing of the fuel assembly. Assembly bowing can lead to undesirable consequences which prudent designers wish to avoid.
Another prior art arrangement locates the holddown mechanism in the assembly lower end fitting. In one such arrangement the lower end fitting has four alignment posts adapted to be located adjacent to four respective alignment pins projecting from the core support stand. Springs connected to the end fitting extend through two proximate posts and are adapted to engage shoulders on the projections. The spring locking surface is oriented to provide vertical holddown of the assembly and to provide horizontal bracing to preclude lateral vibration. The weight of the assembly provides the locking force but large external lifting forces are required to overcome the locking forces in order for the assembly to be removed from the stand. This prior art arrangement was deficient in one major respect. The spring force required to provide sufficient holddown and horizontal bracing was so large that it caused premature deterioration on the shoulders of the support stand projection during fuel assembly disengagement operations. It is preferable that, if one of the locking members is to wear, it should be the member connected to the fuel assembly, since the assembly life is typically 3 or 4 years, whereas the lifetime of the support stand projection is expected to be over 40 years.